Modelled those to center and reduce the excessive drag of a small and heavy spool. Worked for me, so I thought maybe somebody else could find it useful.

Just measure your spool and holder and edit the parameters accordingly (Modify > Change Parameters). Those should be self explanatory.

Want a single adapter covering the whole spool hole, you got it. Want a couple small ones to use with a cardboard spool or just to print it faster and cheaper? Easy. Just choose your flavour, drop the numbers and print. 

Remember to adjust diameters according to your printer tolerances and desired tightness. I do recommend leaving some play on the inner diameter so you can slide the adapter in the spool holder with ease (0.1-0.2mm worked well for  me).

It's probably a good idea to print a slim draft to test measures before printing the whole thing.